A modern optical communication system utilizes optical amplifiers to amplify wavelength division multiplexed (WDM) signal channels as they are transmitted through the system. These amplifiers may be placed at the receiving and transmitting ends of the system, as well as between the various transmission fiber spans comprising the system.
An optical amplifier is characterized, amongst others, by its gain and its noise figure (NF), the latter quantifying the noise introduced by the amplifier into the system. In the case of a WDM system, where the WDM signal channels occupy a signal wavelength band, the optical amplifier is also characterized by the spectral dependence of the gain and the NF within this band. Three main quantities are of interest: (1) the average gain, defined as the gain averaged over the wavelength band; (2) the gain tilt, defined by performing a linear fit to the spectral gain curve over the wavelength band, and calculating the gain difference between the linear fit at the long wavelength (so called “Red”) end of the band, and the linear fit at the short wavelength (so called “Blue”) end of the band; and (3) the maximum NF, defined as the maximum value of the NF over the wavelength band. Unless specifically stated otherwise, the term “NF” used herein will be taken to mean the maximum NF. Unless specifically stated otherwise, all values of gain, gain tilt, attenuation and NF are assumed to be in decibel (dB) units.
In many cases it is beneficial for an optical amplifier to have variable gain functionality. This means that the average gain of the amplifier can be dynamically set to any value within a specified range of values, while at the same time maintaining the gain tilt within required specifications for any setting of the average gain. This variable gain functionality allows the same type of amplifier to be used for different systems and at different locations within a system which require different levels of average gain. In some cases it is also desirable to be able to dynamically set the gain tilt within a certain range of values independently of the average gain.
One type of optical amplifier is a lumped amplifier, which is a completely self-contained unit with well-defined input and output ports, and in which the entire amplification process taking place within the unit. The most commonly deployed example of a lumped optical amplifier is an Erbium doped fiber amplifier (EDFA), which contains at least one length of Erbium doped fiber (EDF) and at least one pump laser diode. The EDF serves as the gain medium which transfers energy from the pump laser diodes (or diodes) to the optical signal channels as they pass through the amplifier, thus providing signal amplification. A lumped amplifier may contain a variable optical attenuator (VOA), which provides variable gain functionality by allowing the average gain to be adjusted by controlling the VOA. In general, increased VOA attenuation results in decreased average gain. In most practical cases, in order to increase the efficiency of the amplifier, the VOA is placed between two gain stages within the amplifier, rather than at the amplifier output. One result is that an increase in the VOA attenuation also results in an increase in the amplifier NF, due to the extra loss imparted to the signal channels. Thus, the amplifier NF increases as the average gain decreases. In many cases it is also possible to independently adjust the gain tilt and the average gain by jointly controlling the VOA attenuation and the pump power used to pump the amplifier. For example, in a variable gain EDFA designed for the C-Band, increasing the VOA attenuation while adjusting the pump power to maintain a constant average gain will decrease the gain tilt (make it more negative). Conversely, decreasing the VOA attenuation while adjusting the pump power to maintain a constant average gain will increase the gain tilt (make it more positive).
In recent years, a new type of amplifier, namely a distributed Raman amplifier (also referred to herein simply as “Raman amplifier” or in short “RA”), has been introduced into optical communication systems. A significant difference between a lumped amplifier and a Raman amplifier is that for the latter, the transmission fiber itself serves as the gain medium, meaning that the signal channels are amplified as they travel through the transmission fiber. Thus, the Raman amplifier itself just provides Raman pump power and control functionality, while the actual amplification process takes place in a distributed manner along the transmission fiber. The distributed nature of the amplification improves the optical signal to noise ratio (OSNR) of the system compared to the case where an equivalent lumped amplifier (such as an EDFA) is used. This is reflected by the fact that a Raman amplifier typically has a lower NF than an equivalent lumped amplifier. As a general rule, the higher the average gain provided by a Raman amplifier, the lower its NF.
The average gain of a Raman amplifier can be adjusted by controlling the amount of Raman pump power injected into the transmission fiber. Furthermore, if the Raman amplifier includes at least two pumps with different wavelengths and means to separately control the power emitted by each pump, then the spectral shape of the gain may also be controlled to a certain degree. Thus, variable gain functionality can be achieved and the gain tilt may also be adjusted independently of the average gain. Raman amplifiers including average gain control and gain tilt control are known.
While Raman amplifiers typically have lower NF than equivalent lumped amplifiers, they are usually restricted in the amount of gain they provide, mainly due to the fact that higher gain requires higher levels of Raman pump power to be injected into the transmission fiber. This both increases the cost of the system and also increases the chance for potential safety hazards and damage related to the propagation of very high levels of pump power in the transmission fiber. Furthermore, very high values of Raman gain are also associated with high levels of double Rayleigh backscattering, which can detrimentally affect system performance.
To overcome this problem, a Raman amplifier is often used in conjunction with a lumped amplifier such as an EDFA, thus forming a so called hybrid Raman-lumped amplifier (also referred to herein simply as “hybrid amplifier” and in short “HA”). In this configuration the Raman amplifier is used as a pre-amplifier while the lumped amplifier is used as a booster amplifier. Since the total NF of an amplifier is usually dominated by the NF of the pre-amplifier, the HA benefits from the low NF of the Raman amplifier. On the other hand, the lumped amplifier booster can provide the extra gain not provided by the Raman amplifier, thus allowing the total gain of the hybrid amplifier to be as high as required.
Some degree of variable gain functionality for a HA can be achieved through control of the gain of the Raman amplifier, as described above. However, since the total gain imparted by the Raman amplifier part of the HA is usually limited, and since it is often difficult to control a Raman amplifier operating at very low gain, the total dynamic gain range achievable using this method is limited. Thus, it is often beneficial for the lumped amplifier part of the hybrid to provide variable gain functionality as well, thus improving the variable gain functionality of the HA as a whole. In such a case, a key question is how to divide the total gain of the HA between the Raman amplifier and the lumped amplifier, while at the same time minimizing the overall NF of the HA. A known method for achieving this is by storing a database of the HA NF as a function of the Raman amplifier gain for different values of total HA gain. Thus, for a given required total HA gain, the Raman amplifier gain that provides the minimum NF can be found and set accordingly. Then, the lumped amplifier average gain is set to provide the remaining gain needed to achieve the required total HA gain. However, this method does not account for the spectral dependence of the gain in WDM systems, and therefore does not take into account the added degree of freedom which is available through controlling the gain tilt (not just the average gain) of the Raman amplifier and lumped amplifier.
Another known solution implements optimization of the spectral shape of the Raman amplifier gain in order to optimize the HA NF. This is achieved by providing a higher Raman gain in the spectral region where the lumped amplifier has the highest NF, thus reducing the total HA NF in that spectral region. However, the total gain of the HA NF is assumed to be fixed, so that the optimization of the NF in the case of a variable gain HA is not considered at all.
There is therefore a need for, and it would be advantageous to have, a variable gain hybrid amplifier and methods for optimizing the NF in a variable gain hybrid amplifier, in which both the average gain and gain tilt of the Raman amplifier and the lumped amplifier can be controlled independently.